The aim of the proposed studies is to study the firing behavior of afferent inputs to the vestibular nuclei that are influential in modifying signal processing in vestibulo-ocular reflex (VOR) pathways during smooth tracking head movements and during near viewing. The initial studies will focus on the contribution of cerebello-vestibular pathways to these functions. Other possible sources of vestibular nucleus afferent inputs that modify signal processing in VOR pathways as a function of behavioral context will be considered if the cerebello-vestibular pathways do not appear to be sufficient to suppress the VOR during active head movements or for modifying signal processing in VOR pathways during near viewing. The first specific aim is to determine the source and characteristics of the head movement efference copy inputs to the vestibular nuclei that cancel or attenuate vestibular signals on secondary VOR neurons during gaze pursuit. Monkeys will be trained to pursue targets moving in the horizontal plane using smooth movements of the eye, head and/or body. Unit responses during active and passive movements will be compared. Regions of the cerebellum and brainstem that contain cells that are differentially sensitive to active head movements will be investigated further using electrical stimulation and chemical inactivation techniques. Anatomical studies will be carried out using retrograde tracers in combination with localized marking of recording sites to determine whether regions containing active head movement cells also project to the vestibular nuclei. The second specific aim is to determine the contribution of the cerebellar flocculus to viewing distance related changes in the angular and linear VOR during passive and active head movements. Monkeys will be trained to fixate or pursue visual targets presented on an earth stationary display mounted on a motorized track so that its distance can be varied from 10-150 cm. Vestibular stimuli will consist of passive whole body angular rotation or linear translation in head-restrained monkeys. Single unit recordings will be obtained from flocculus Purkinje cells and secondary VOR neurons. Unit responses to passive and active whole body motion at different viewing distances and target eccentricities will be recorded. The contribution of the flocculus to viewing distance related changes in the VOR will be assessed by examining the viewing distance related changes in the responses of Purkinje cells and VOR pathway neurons. Vestibular nucleus neurons that receive inputs from the cerebellar flocculus will be identified by electrical stimulation of the flocculus. Particular attention will be paid to determine whether different Purkinje cells are differentially sensitive to the movements of one or both eyes or to heave or thrust translation. The neurophysiological observations will be evaluated in the context of the observed effects of unilateral or bilateral inactivation of the cerebellar flocculus on the angular and linear VOR. The results should increase our understanding of the central mechanisms that allow clear vision of objects in a 3D environment during passive and active head movements.